1. Field of the Invention
This invention relates to a method for coating a metal cookware, more particularly to a method for coating a metal cookware in order to provide an improved anti-stick layer on a cooking surface thereof so as to enable the metal cookware to have a relatively long service life.
2. Description of Related Art
U.S. patent application No. 08/055,680 discloses a method for coating a metal cookware so as to provide a scratch-resistant and anti-stick cooking surface thereto. The method includes the steps of: (a) roughening and cleaning a cooking surface of the metal cookware by blasting aluminum oxide particles against the cooking surface; (b) applying a metallic scratch-resistant layer onto the roughened and cleaned cooking surface by electric-arc spraying; and (c) applying an anti-stick layer, consisting essentially of polytetrafluoroethylene (PTFE), onto the scratch-resistant layer by spray coating until the anti-stick layer has a thickness of about 23-36 .mu.m, whereby a pencil hardness of 8-9 H is achieved.
Although the metal cookware which is coated as such has a scratch-resistant and anti-stick cooking surface, it has been found that the cooking surface rusts easily. This is due to the fact that PTFE has a crystallinity of over 99%, which tends to create pores and cleavages in the anti-stick layer after a sintering process, thereby exposing the metallic scratch-resistant layer to the atmosphere. Thus, the coated metal cookware cannot have a very long service life.
There are other prior art references concerning the technique of preparing and applying PTFE coatings. Examples of them are U.S. Pat. Nos. 3,900,684, 4,070,525, 4,196,256, 4,252,859, 4,395,445, 4,568,573, 4,618,511, 4,683,168, 4,702,933, 4,808,490 and 4,862,609.